It is well known to provide electronic article surveillance systems to prevent or deter theft of merchandise from retail establishments. In a typical system, markers designed to interact with an electromagnetic or magnetic field placed at the store exit are secured to articles of merchandise. If a marker is brought into the field or "interrogation" zone, the presence of the marker is detected and an alarm is generated. Some markers of this type are intended to be removed at the checkout counter upon payment of the merchandise. Other types of markers are deactivated upon checkout by a deactivation device which changes an electromagnetic or magnetic characteristic of the marker so that the marker will no longer be detectable at the interrogation zone.
It is a common practice for the presence of the marker to be detected in the interrogation zone by detecting a signal reradiated by the marker in response to the field present in the interrogation zone. For example, U.S. Pat. No. 4,063,229 issued to Welsh et al., discloses several types of markers which generate harmonic signals in response to an alternating field provided in the interrogation zone. The Welsh et al. patent suggests that such markers may be integrated with a price label adhesively attached to an article of merchandise or that one or more markers may be imbedded or incorporated in the packaging for the article or in the article itself.
Other types of harmonic EAS systems are based on markers which include a thin strip or wire of magnetic material that responds to an alternating interrogation signal by generating a signal pulse that is rich in high harmonics of the interrogation signal. Such markers are disclosed in U.S. Pat. No. 4,660,025 to Humphrey and U.S. Pat. No. 4,980,670 to Humphrey et al.
Another type of EAS system employs magnetomechanical markers that include a magnetostrictive element. For example, U.S. Pat. No. 4,510,489, issued toAnderson et al., discloses a marker formed of a ribbon-shaped length of a magnetostrictive amorphous material contained within a hollow recess in an elongated housing in proximity to a biasing magnetic element. The magnetostrictive element is fabricated such that it is mechanically resonant at a predetermined frequency when the biasing element has been magnetized to a certain level. At the interrogation zone, a suitable oscillator provides an AC magnetic field at the predetermined frequency, and the magnetostrictive element mechanically resonates at this frequency upon exposure to the field when the biasing element has been magnetized to the aforementioned level. The resulting signal radiated by the magnetostrictive element is detected by detecting circuitry provided at the interrogation zone. The Anderson et al. patent points out the need to form the housing for the marker so that the mechanical resonance of the magnetostrictive element is not mechanically damped. Anderson et al. also teach that the marker should be formed so that the biasing magnet does not mechanically interfere with the vibration of the magnetostrictive element. The disclosure of the Anderson et al. '489 patent is incorporated herein by reference.
EAS systems which use magnetomechanical markers have proved to be very effective and are in widespread use. Systems of this type are sold by the assignee of this application under the brand name "Ultra*Max". In operating such systems, it is customary to attach magnetostrictive markers to the items of merchandise at retail stores which maintain equipment for generating the field for the interrogation zone. The attachment of the markers to the items of merchandise is typically carried out by means of a pressure sensitive adhesive layer provided on the marker, or, when the marker is intended to be removable, by a mechanical clamping device or the like. One example of such a device is disclosed in U.S. Pat. No. 5,031,756, issued to Buzzard et al., which is directed to a "keeper" which may be utilized in a retail store. The keeper includes a frame for holding a compact disk or similar item, and the compact disk may be locked within the frame to prevent removal of the compact disk from the keeper until the compact disk is paid for at a checkout counter. The keeper disclosed by Buzzard et al. includes an EAS marker which may be a magnetomechanical marker of the type described in the Anderson et al. patent.
In order to improve the efficiency of operation of retail establishments, it has been proposed that EAS markers, including magnetomechanical markers, be applied to the items of merchandise before shipment to the retail establishment. For example, it has been proposed that markers be attached to the goods by manufacturers thereof. This practice has been called "source tagging," which means that an EAS marker or "tag" is applied to goods at the source of the goods. While conventional techniques for attaching markers to goods, which include attaching markers to goods by means of adhesives, have been proposed for use by manufacturers, it would be desirable to provide still more efficient techniques for "source tagging" goods that will ultimately be subject to electronic article surveillance at a retail establishment. Although the Welsh et al. patent suggests that certain kinds of harmonic signal generating markers could be physically embedded in a product or product packaging, that patent is not concerned with the type of marker used in magnetomechanical EAS systems and does not address how the elements making up such a marker could be embedded in a product without constraining the mechanical resonance of the magnetostrictive element and thereby preventing the marker from operating.
The following U.S. patents also propose incorporation of marker elements within an article to be subjected to electronic surveillance:
U.S. Pat. No. 3,665,449 to Elder et al., which discloses embedding a ferromagnetic strip in a library book. PA1 U.S. Pat. No. 4,151,405 to Peterson, which discloses embedding ferromagnetic strips in plastic, paper, wood, aluminum, stainless steel, etc. PA1 U.S. Pat. No. 4,626,311 to Taylor, which discloses embedding marker elements in a thermoplastic holder which is then fused within a garment. PA1 U.S. Pat. No. 4,686,154 to Mejia, which discloses concealing a tag within a seam or lining of an article of clothing. PA1 U.S. Pat. No. 4,835,028 to Dey et al., which discloses a magnetostrictive wire embedded in paper.
However, like the Welsh et al. patent, none of these patents is concerned with magnetomechanical markers and none teaches how the elements of such markers could be embedded in a product without constraining the mechanical resonance of the magnetostrictive element.
Moreover, the prior art also fails to teach how to embed in a product magnetic elements like those disclosed in the above-referenced Humphrey and Humphrey et al. patents. U.S. Pat. No. 4,342,904 proposes a marker structure that includes release sheets surrounding the ferromagnetic material within the marker structure to prevent or minimize transfer of stresses to the ferromagnetic material, because such stresses tend to "cold work" the ferromagnetic material and degrade its magnetic properties. Similarly, it is known to apply a lubricant to the type of magnetic material disclosed in the Humphrey '025 patent before forming a marker by laminating flexible sheets around the material. The lubricant prevents stress from being applied from the surrounding sheets to the magnetic material when the marker including the magnetic material is applied to a product. However, it has not heretofore been recognized that embedding the Humphrey or Humphrey et al. material in a product would also tend to produce stresses on the material that would degrade its performance.